Research topics

The group "Molecular Kinetics and
Spectroscopy" of Martin Quack at ETH has as main research theme the
understanding of fundamental, physical-chemical molecular primary processes.
This research can be broadly classified in the following more detailed
subfields.

1. Intramolecular kinetics, vibrational redistribution, and
energy
transfer in polyatomic molecules and hydrogen
bonded clusters
from the sub-femtosecond to nanosecond time scales

as
derived by analysis of high resolution infrared spectroscopy using an
"indirect" experimental approach pioneered by the Quack group (making use of
high frequency resolution and broadband coverage, but not high time
resolution). This approach has shown that intramolecular rate processes of
vibrational redistribution are often governed by specific properties of
functional groups and thus can be transferred from one molecule to another.
For instance in the alkyl CH chromophore redistribution occurs in 100 fs or
less, whereas it is longer than ps in the seemingly similar acetylene chromophore or the hydrogen bonded (HF)2
molecules and related larger clusters (HF)n. Furthermore this
approach has shown highly mode specific tunneling dynamics for hydrogen bond
switching in (HF)2 isotopomers as well
as prototypical stereomutation dynamics in simple chiral molecules such as H2O2
or in CH3OH and NH3 isotopomers. These researches have
fundamentally changed our thinking on intramolecular energy transfer in
polyatomic molecules, replacing the earlier well established dogma of
universally fast intramolecular equilibration on the subpicosecond time
scale, which is the basis of statistical theories of chemical reactions
(Quasi-equilibrium theory, QET, RRKM theory, transition state theory,
statistical adiabatic channel model, SACM). Reviews: e, f, h, i, l, m, o, p,
q, r, t (see below).

Polyatomic molecules in the gas phase under
intense infrared radiation from pulsed lasers can absorb many photons,
typically 10 to 50, but even up to several hundred, in the case of C60.
Subsequent to this high vibrational excitation they will often react by
decomposition or isomerisation, exceptionally, as in the case of C60,
by ionisation. In this research fundamental, quantitative aspects of these
processes are studied in relation to a general quantitative theory
originally developed by M. Quack in the 1978 – 82 period.
Quantitative experimental tests of the theory are accompanied by
applications of selective laser chemistry such as 13C/12C
isotope separation, studies of state selected kinetics including hyperfine
resolution and related processes. One long-term goal of the research is the
development of tools for mode selective chemistry using also results from
the research described under section 1. Infrared chromophores are
systematically studied in relation to the "chromophore principle" of
laser chemistry. Infrared laser chemistry has also some promise for possible
industrial applications. Reviews: a, b, d, g, k, q, r, t.

3. Spectroscopic studies of time resolved reaction kinetics
on the fs to μs time scale

complement
the research discussed under sections 1. and 2.
Recent advances should allow us to bridge the gap between our spectroscopic
studies, which have in fact provided the first nontrivial, multidimensional,
experimental femtosecond wavepacket dynamics, and the more widely used
standard approaches of femtosecond kinetics by the well known and widely
used pump-probe techniques. Reviews: k, q, t.

4. Development of high resolution infrared spectroscopic
methods for studies of intramolecular kinetics, atmospheric chemistry, and
hydrogen bond cluster structure and dynamics

In these researches experimental tools are
developed which allow for the analysis of very complex vibration – rotation
spectra of polyatomic molecules. The application of
these techniques range from the fundamental studies mentioned under section 1.to spectroscopic investigations of atmospheric
pollution within an AGS project (Alliance
for Global Sustainability). A major stumbling block in
infrared gas phase spectroscopy in the past has been the inability to
quantitatively analyze the complex very rich line spectra of polyatomic
molecules with several "heavy" (i.e. non hydrogen) atoms. The Quack group
has systematically developed experimental methods allowing for such
analyses. Among these techniques we may mention the development of high
resolution Fourier Transform Infrared – Supersonic Jet Spectroscopy over
several "generations" of experimental apparatus, which allows simplification
of spectra through jet cooling. Currently the worldwide highest resolution FTIR (Bruker prototype) spectrometer (resolving power 2 x 106,
experimental bandwidth 0.0007 cm-1) is located in the laboratory
of the group, in addition to an older BOMEM prototype system (0.004 cm-1
bandwidth, full width half maximum, apodized). Another recent development is
the still worldwide unique combination of tunable cw-diode laser cavity ring
down spectroscopy, allowing for a resolving power of 2 x 108 in
the near infrared (experimental bandwidth about 1 MHz) with pulsed
supersonic slit jet expansions. Further techniques applied in the group are
lead-salt diode laser spectroscopy with supersonic jets and time resolved
step-scan FTIR spectroscopy (at modest spectral resolution).

To demonstrate the need for such
developments we may mention here just three out of many examples. Firstly,
for atmospheric applications the important "atmospheric window" band of the
Freon CFCl3 could not in the past be rovibrationally analyzed in
spite of many attempts. This problem was solved, using the techniques
mentioned, by our group in a publication in 1995. Secondly for the whole
class of chiral molecules (of necessarily a certain minimum complexity)
there existed in the past (until about 1995) not a single case, where a
rovibrational line analysis of an infrared gas phase band had been achieved.
Today we have a few such analyses from recent work in our group (still the
only ones worldwide, to our knowledge) such as CHFClBr, CDFClBr,
Fluorooxirane CHFCH2O, Dideuterooxirane (CD2CH2O),
Difluoroallene, CHF=C=CHF, PF35Cl37Cl etc. Thirdly,
the complex and weak overtone polyad absorption of (HF)2
in the 7700 cm-1 near infrared spectral range could only
incompletely be analyzed in the past in spite of several attempts. Our group
has recently achieved a full analysis of the polyad using the techniques
mentioned. Reviews: e, m, q, t.

We continue our long-term theoretical
investigations on chemical reactions. The developments range from full (up
to 6-dimensional) quantum wavepacket dynamics in simple (up to four atoms)
reactions such as H2O2 stereomutation, (HF)2
hydrogen bond dynamics, inversion in NHD2, and related problems.
We furthermore test and develop higher dimensional approximate techniques
such as the quasiadiabatic channel reaction path
hamiltonian method for tunneling reactions (for instance in CH3OH
or aniline-NHD, C6H5NHD etc.). We treat the full
quantum dynamics of coherent infrared excitation. At the same time we
develop approximate quantum statistical master equation approaches, which go
beyond quasi-equilibrium statistical theories (RRKM, QET, SACM). The latter
theories are also further developed in some of our work. Reviews: a, j, k,
l, m, p, q.

Starting with a fundamental paper on
detailed symmetry selection rules in chemical reactions based on the
principles of approximate nuclear spin symmetry and parity conservation in
molecular processes (Mol. role of fundamental symmetry
principles in molecular physics. Recent work has focussed on parity
violation in chiral molecules. We have shown in recent years, that parity
violating energy differences between enantiomers of chiral molecules are
predicted by our newly developed theories to be typically one to two orders
of magnitudes larger than anticipated on the basis of older theories. These
findings have now also been confirmed by other groups and have important
consequences for our attempts to measure this still very small energy difference (ΔRH0≈ 10-11J mol-1) calculated for CHFClBr, for example. A combination of
experiment and theory in the future will be important for insights into the
"Standard Model", a general theory of high energy particle physics. Indeed,
the proposal for the most sensitive test of CPT symmetry so far made (with
Δm/m =10-30compared to other current proposals and actual tests
in the 10-10to 10-20range) stems from this work,
although our investigations here can only be of preparatory nature, the
final experimental CPT tests being envisaged using our spectroscopic
developments in conjunction with high energy physics facilities such as
CERN. We should make clear that this last aspect, while being of
intellectual, scientific interest, has no practical short-term prospect. On
the other hand, molecular parity violation in itself is of fundamental
interest and actively investigated by the group. Reviews: c, f, n, s, t.

7. Useful reviews for more detailed overviews and summaries
of the research of the Quack group for molecular kinetics and spectroscopy

Many aspects of the research can be found in the articles authored by the group in the Handbook of High Resolution Spectroscopy edited by
Martin Quack and Frédéric Merkt